#include<stdio.h>
void main()
{
	float mu = 1;

	float sum = 0;

	int flag = 1;

	while (1/mu>=0.0001)
	{
		sum += flag/mu;

		mu += 2;

		flag = -flag;
	
	
	
	}

	printf("%f\n",sum*4);


}
